The present invention relates to a method of improving the sludge volume index (SVI) of mixed liquor in the aeration tank of an activated sludge plant.
Wastewater such as sewage that contains organic matter is conventionally treated by the activated sludge process. The activated sludge plant basically consists of an aeration tank and a sedimentation tank, and the wastewater is first supplied to the aeration tank. The aeration tank contains several thousand ppm of suspended solid populated by microorganisms that oxidatively decompose the organic matter in the wastewater to carbon dioxide and water. The effluent from the aeration tank is subjected to solid-liquid separation in the sedimentation tank and the supernatant is either discharged into water courses or recycled for further use. Part of the settled sludge is returned to the aeration tank and the remaining sludge is discharged from the plant as excess sludge. While the activated sludge plant is operated in this manner, sludge bulking sometimes occurs in the sedimentation tank. If sludge bulking occurs, the solids content of the settled sludge drawn from the sedimentation tank is decreased and the content of the solids in the aeration tank is also decreased to make the process operation difficult due to overload. What is more, part of the sludge overflows the sedimentation tank to impair the quality of the purified water.
The causes of the sludge bulking have not been fully unravelled, but it is generally ascribed to poor coagulation of the sludge due to increased growth of filamentous organisms such as Sphaerotilus, and to the swelling of bacteria in the sludge due to bound water. An overload on the aeration tank due to increased concentration of the organic matter in the wastewater or increased volume of the water flowing into the aeration tank is said to cause the increased growth of filamentous organisms, hence sludge bulking. Other factors considered responsible for sludge bulking are increased hydrogen sulfide, unbalanced nutrient salts and insufficient aeration.
Sludge bulking is conventionally prevented by supplying more air into the aeration tank or by providing a detention reservoir before the aeration tank to make the load on the aeration tank uniform over time. If these methods prove to be ineffective, sterilizers such as chlorine, copper compounds, ozone and chloroform to kill filamentous organisms are added to the aeration tank or the returned sludge, or a flocculation agent is added to coagulate the sludge.
Since sludge bulking is an unpredictable phenomenon addition of the sterilizer or flocculation agent must be done continuously to completely prevent the phenomenon. But to add these chemicals over an extended period of time results in an increased operation cost. As a further disadvantage, the sterilizer kills not only the filamentous organisms but also the other microorganisms in the system, so if it is added in excess, the activity of the microorganisms is decreased and the quality of the purified water impaired.
Japanese Patent Application No. 164236/79 discloses a method without a sedimentation tank wherein a cationic polymeric flocculation agent is added to the effluent from the step of biologically oxidizing organic wastewater, the effluent is dewatered by centrifuge the dewatered cake is mixed with another effluent from the biological oxidation step to form a slurry, which is returned to the aeration tank. Japanese Patent Application No. 72485/80 discloses a method wherein an oxidizer and inorganic flocculation agent are added to concentrated excess sludge, the sludge is dewatered by a filter press, the separated water is recycled for use as a flocculation agent in a coagulation/sedimentation step before or after the biological treatment step. Wlb-wasser luft und betrieb-22 (1978) Nr 10, pp. 530-533 discloses a method that improves the settleability of swollen sludge by shredding it with a centrifuge. The first and third methods that rely on concentration by centrifugal force are effective in increasing the solids content of sludge, but at the same time, they have negative effects on floc formation since they cut the sludge into shreds. In addition, the first method intends a biological treatement with the step of gravitational settlement whereas the present invention aims at improved SVI by separating the sludge through gravitational settlement. The second reference teaches only a method of reusing the flocculation agent in the water separated from the concentrated excess sludge. Therefore, none of these references disclose a technique of the type comtemplated by the present invention that achieves improved SVI by increasing the ability of the particles of the sludge to coagulate and form flocs of high density, as well as by increasing the dispersibility of the concentrated sludge in the aeration tank.